Quality control is extremely important in the food service industry. Food manufacturers spend substantial amounts of money to ensure that their packaged nutritional products, including pediatric and medical nutritional products, are not contaminated.
There are two distinct methods of sterilization currently in use. The first method involves the use of a retort process. Utilization of this method features the introduction of the food product into a container, which container then is subjected to extremely high retort temperatures which effectively kill all the micro organisms which may have been in the sealed unsterilized container. Once a food product has been subjected to the retort process, it should be commercially sterile, provided there are no problems with improper processing or container damage during the sterilization process.
The second type of sterilization involves aseptic processing and packaging. Aseptic processing and packaging has become established as a feasible method for the manufacture of sterilized nutritional product. However, the microbiological quality of products manufactured using this technique requires strict control. Under this process, sterile product is brought into contact with a sterile container in a sterile zone. The sterile zone has an air overpressure with sterile air that is usually HEPA filtered.
The key advantage associated with the aseptic method concerns the improved nutritional content of the product. Subjecting a nutritional product to retort or terminal sterilization can result in a change of flavor, or a change in color under such high heat treatment. Although aseptic packaging overcomes such problems, sterility becomes more of an issue. This is because products packaged using the aseptic process are more suspectible to contamination because of the nature of the process.
The key disadvantage associated with aseptic processing and packaging is that the presence of even one microbe in an aseptic packaged product may result in the contamination of the entire product, whereas the presence of one microbe in a sealed container subsequently subjected to retort usually results in the killing of the microbe.
A tacit acknowledgment of the problems associated with aseptic packaging is the fact that oftentimes, aseptically packaged nutritional products are placed in a refrigerated environment. Although in theory the fact that the product has been aseptically packaged should result in the product not spoiling, companies which provide nutritional products packaged using aseptic procedures will recognize the fact that if even one microbe is introduced into the packaged material, then contamination can exist. Therefore, by keeping the nutritional product in a refrigerated environment, hopefully any such contamination, if it exists, will be minimized. The concern is heightened as aseptic processing becomes more low acid. Thus, it can be appreciated that the need exists for a non-destructive testing of an aseptic packaged product in order to further provide assurance that the nutritional product is not undergoing spoilage.
Quality control is extremely important as it relates to the sterility of nutritional products. A number of various methods of detecting spoilage exist; however, there are significant drawbacks associated with them. As used herein and in the claims, a non-destructive method of detecting spoilage is a method which does not require the opening of a container in order to test the nutritional product contained therein. A destructive method is one which requires the opening of the container in order to test the nutritional product contained therein.
One prior art test for sterility is the pH drop test. This test measures the nutritional product for evidence of a drop in the pH between the time when the product produced and the time when the product is tested. Evidence of a drop in pH often indicates that the product is not sterile and that the product is encountering the growth of bacteria or other microorganisms. There are two disadvantages associated with pH drop tests. The first is that the test is a destructive test. The second disadvantage is that this test is typically conducted, due to time constraints, on a limited number of containers per batch. This method is not routinely used for commercial product batches. It is usually incorporated for production line start up procedures or new product start up.
Another test for sterility involves a visual inspection of the product, provided the container walls are either transparent or at least translucent. The obvious disadvantages associated with this test include the fact that sometimes contaminated product will not appear visually different than sterile product, that this method is labor intensive and consequently inefficient in testing large numbers of containers, and that if the container is made of metal, is not translucent, or has a label affixed to the container such that the nutritional product is not visible, the results of any attempted visual inspection of the product leave a great deal to be desired.
Yet another test is known as the BACTEC sterility test. This test utilizes a subculture method that evaluates a small batch sample, typically numbering eight to ten per batch or sterilizer load. In practice, the fact that a subculture does not indicate a problem is interpreted as evidence that the entire batch or load has no problems with sterility.
Still another test is known as the leak test, which only indicates the potential of a non-sterile package. This test is conducted in a vacuum and detects the presence of flush gas immediately after the container comes off the sterilization line. In theory, testing for the presence of a flush gas, such as nitrogen in the case of aseptic packaging, will provide an indication that there is a leak in the container which could facilitate contamination. In reality though, this test is not fool proof.
Another test involves spinning the container for indications of an increase in viscosity of the product. However, interpreting the results of this test is often too subjective. Yet another test is known as the PECO test, which detects the presence of a dome being formed in the container due to spoilage. However, often the container may appear fine, while the nutritional product is actually contaminated.
Yet another method of testing for sterility involves shaking of the container. Once again, the theory is that any gelation which occurs due to contamination will be felt in the shaking process. However, since not all spoilage results in gelation, this test is of limited applicability. Furthermore, the test is extremely subjective.
Given the problems associated with the many tests which have been utilized in connection with testing for sterility, manufacturers of nutritional products have endeavored to develop more efficient tests. An example of this research resulted in an attempt to apply ultrasonic imaging to the assessment of micro-biological quality of aseptically packed starch soup. Research concluded that a non-destructive method using ultrasound imaging resulted in a method appropriate for use in quality control schemes for starch-based foods. However, ultrasound testing has several disadvantages. First, the ultrasonic method is very dependent on gelation or viscosity-build up. This is the principal behind its ability to detect spoilage. Second, the method is somewhat time consuming, as well as relatively expensive. Third, this method of testing requires either good acoustic contact with, or water immersion of, the container.
The present invention relates to the utilization of nuclear magnetic resonance spectroscopy (NMR) to detect spoilage in a nutritional product. Nuclear magnetic resonance spectroscopy is based on the measurement of resonant radio frequency adsorption by nuclear spins in the presence of an applied magnetic field. In NMR an object is placed in an applied static magnetic field. The nuclei in the molecules of the sample generate a bulk microscopic magnetization. Once generated, this magnetization can be perturbed by a second field which is oscillating at an appropriate radio frequency. This perturbation of the magnetization generates the nuclear magnetic resonance spectrum. There are two basic types of NMR, the wide-line or frequency-sweep method and the pulsed or transient response method. The frequency-sweep method is analogous to tuning a piano note by note, striking each note and listening to the response. In continuous wave spectroscopy, the disturbance of the magnetization is monitored as either the radio frequency is varied or the applied field is swept. In each case only a single frequency is excited and detected at any one moment. In pulsed nuclear magnetic resonance spectroscopy (PNMR) the pulsed measurement is comparable to all the keys of a piano being struck at once, with the response of each note extracted from the total sound.
In pulsed NMR a large permanent magnetic field is applied to a sample. The magnetic field should be as homogenous as possible so as to apply a uniform magnetization to the sample. A pulse is applied by a rotating magnetic field at right angles to the large permanent field. This causes the spin nuclei being probed to become aligned in a single direction. The rotating field is generated through a tuned coil and consists generally of a burst or pulse of radio frequency energy. This radio frequency pulse rotates the magnetization of the sample. After the pulse has been applied, the spin system tends to return to its equilibrium position in the large permanent field through dephasing. After the equilibrium magnetization is re-established, the radio frequency pulse may be repeated. The induced voltage generated by the spin dephasing process is monitored by a resonant radio frequency coil.
An important aspect of nuclear magnetic resonance spectroscopy is the signal known as a free induction decay (FID). FID is caused by the magnetization of the object returning to equilibrium, and the consequent "decaying" of the NMR signal. The decaying signal is also referred to as a "transient". An FID signal can readily be ascertained from the PNMR method. Additionally, with modern instruments and computers it is also possible to obtain an FID signal from the frequency sweep method by performing an inverse Fourier Transform (FT) on the data generated.
The theory and application of NMR in connection with nutritional products focuses on particular qualities associated with nutritional products. A major advantage of NMR is that it is neither invasive nor destructive. Consequently, nutritional products still sealed in their original non-magnetic containers, such as glass or plastic bottles, may be analyzed without detriment to either sterility or wholesomeness. For example, NMR has been utilized to facilitate measurement of the influence of dispersion and composition on fat nucleation and crystallization in food such as biscuits, emulsions and confectionery products. Such tests enable the manipulation of the content and physico-chemical properties of fat so as to optimize taste, texture and nutritional quality of various foods. "Magnetic resonance imaging application in food research", McCarthy et al., TRENDS IN FOOD SCIENCE & TECHNOLOGY, December 1990, pp. 134-139.
Yet still further, NMR has been used for the estimation of fat content in pork and beef carcasses, determination of water core distribution in apples, for the detection of bruising in apples, onions and peaches, and for the detection of worm damage in pears. Still further, NMR has been utilized to test for the maturity of tomatoes and avocados.
Additionally, NMR has been utilized with respect to oxygen-dependent core breakdown in Bartlett pears and the deterioration of apples during storage. NMR has also been utilized to quantitatively test for oil content in salad dressings. "Use of Magnetic Resonance Procedures for Measurement of Oil in French-style Dressings", Heil et al., JOURNAL OF FOOD SCIENCE, Vol. 55, No. 3, pp 763, 764, 884 (1990). NMR has also been used to evaluate the oxidative deterioration of crude and stored fish oils. "Application of the NMR Method to Evaluate the Oxidative Deterioration of Crude and Stored Fish Oils", Saito, et al., AGRICULTURAL AND BIOLOGICAL CHEMISTRY, 54(2), pp 535-534 (1990). Furthermore, NMR has been applied to measurements of moisture in a variety of products. These include wheat, oats, rice, sugar, starch and its derivatives, candy, corn, skimmed milk powder, and flour.
Despite the varied uses for which NMR and PNMR have been applied, none of them appear to have been related to the determination of spoilage of a nutritional product. Instead, as has been observed above, the usage of NMR or PNMR has been with respect to a quantitative value, such as the percentage of oil in a salad dressing, or the percentage of water in a food product.
It is thus apparent that the need exists for an improved non-destructive method for the detection of spoilage in a nutritional product and the like.